1. Field of the Invention
The invention relates to a power supply device and, more particularly, to a controllable power supply device with a step-up function.
2. Description of the Related Art
Since the present science and technology has made enormous progress, the market have continuously weeded through the old to bring forth the new for electronic devices, and then the requirement that a power supply device has to have a plurality of groups of different voltage levels is more and more universal, and the requirement of the accuracy of each group of the voltage level provided by the power supply device becomes stricter and stricter. For example, the present market has several tens kinds of central processing units (CPU) applied to personal computers. Taking the same manufacture company as an example, operation voltages of the manufactured several kinds of CPUs are not the same, or taking the same type of the CPUs as an example, when a user wants to boost the voltage for some purpose, the power supply device has to provide a plurality of groups of power supply voltages to satisfy the requirement of the user.
FIG. 1 is a structural diagram showing a conventional step-up circuit 100. In FIG. 1, the conventional step-up circuit includes a linear regulator 120, resistances R11˜R16 and switches SW1˜SW4. The linear regulator 120 further includes an operation amplifier 121, N-type transistor T21, resistances R21˜R22 and a capacitor C21.
Please refer to FIG. 1, and the resistances R15 and R16 are serially connected between a voltage VS11 and a grounding terminal to generate an initial voltage VI. The output terminal of the operation amplifier 121 is coupled to the negative input terminal of the operation amplifier 121 via the capacitor C21 to form a buffer circuit. The resistances R11˜R14 can form parallel connection combinations in different coupling modes by switching the switches SW1˜SW4, and form a current path to the ground with the resistances R21˜R22, the transistor T21 and the voltage VS12. Herein, the resistance values of the resistances R21˜R22 are diverse from each other, and then the conventional step-up circuit 100 utilizes the parallel connection combination formed by the resistances R21˜R22 and the resistance voltage division principle to generate a plurality of groups of power supply voltages to achieve the step-up objective.
FIG. 2A is a schematic diagram showing the step-up table of the power supply voltage of the conventional step-up circuit 100. As shown in FIG. 2A, the resistances R11˜R14 have sixteen types of parallel connection combinations via the on or off statuses of the switches SW1˜SW4, in other words, when the step-up circuit 100 works under different step-up orders, the output voltage VO will be boosted to different power supply voltages. For example, if the step-up order is one, the conventional step-up circuit 100 enables the output voltage VO to be boosted to the power supply voltage V1 via the on status of the switch SW1. Similarly, when the step-up order is sixteen, the conventional step-up circuit 100 enables the output voltage VO to be boosted to the power supply voltage V16 via making the switches SW1˜SW4 be on. The magnitude relation among the power supply voltages is that V1<V2<V3 . . . <V16, and then if the step-up order of the conventional step-up circuit 100 is higher, the step-up magnitude is higher.
However, the conventional step-up circuit 100 is still limited by the hardware. For example, the conventional step-up circuit 100 generates a plurality of groups of power supply voltages by the parallel connection cooperation of a plurality of resistances with various resistance values. But if the user needs further more groups of power supply voltages, the conventional step-up circuit 100 has to be cooperated with more resistances, and then the circuit will become further more complex and further huger.
In addition, FIG. 2B is a diagram showing errors of power supply voltages of the conventional step-up circuit 100. As shown in FIG. 2B, voltage errors formed by the power supply voltages are different when the conventional step-up circuit 100 works under different step-up orders. Herein, the voltage errors formed by the power supply voltages become larger along with the increment of the step-up order. The main reason is that the resistance value of the resistances R11˜R14 which are parallelly connected is not linearly decreased, and then the error of the power supply voltage is increased along the increment of the step-up order.